Home Chemistry Element Oxygen


What is Oxygen?

Oxygen, atomic number 8, symbol O, and molecular formula O2 is the most abundant chemical element of the Group-16 or chalcogen family in the periodic table. It makes up 47 percent of the earth’s crust in the form of liquid and solid oxides, oxoacids, and gas forms. Oxygen is essential for living organisms (plants and animals).

Oxygen element chemical symbol and the periodic table properties

The elemental oxygen atom is an isotopic mixture of 16O, 17O, and 18O isotopes. It occurs in the natural environment 21 percent by volume in air, and 86 percent by weight of liquid water in oceans.

Isotopes of Oxygen and Their Uses

Oxygen has three known stable isotopes: 16O, 17O, and 18O. The isotopes 17O and 18O can prepared by fractional distillation or electrolysis of water molecules or thermal diffusion of oxygen gas.

Main isotopes Relative atomic mass Abundance
16O 15.994915 99.763%
17O 16.999134 0.0380%
18O 17.999160 0.205%

Elemental oxygen isotope (18O) is used as the tracer in chemical kinetics and mechanistic studies. The isotope 17O (mass number = 17) is used in NMR spectrum analysis.

Oxygen on the Periodic Table

The nonmetallic chemical element oxygen is placed in group 16 of the periodic table. It is a p-block element placed with sulfur, selenium, tellurium, and polonium. The position of oxygen in the periodic table is given below the picture,

Position of nonmetal chemical element oxygen in the periodic table

Properties of Oxygen

Dioxygen is a powerful oxidizing agent comparable to that of acidified dichromate solutions.

½ O2 + 2 H+ 2 e → H2O (E° = +1.23 V)

The rates of oxidation by oxygen may sometimes be increased remarkably when transition metal ions are used as a catalyst.

Symbol O
Discovery Joseph Priestley and independently by Carl Wilhelm Scheele in 1774
Name derived from The Greek word oxy genes, meaning acid-forming
Allotropes O2 and O3
Common isotope 8O16
Crystal structure Cubic crystal lattice
Periodic properties
Atomic number 8
Electron per shell 2, 6
Atomic weight 15.999
Electronic configuration 1s2 2s2 2p4
Group 16
Period 2
Block p-block
Physical properties
State at 20 °C Gas
Melting point −218.79 °C, −361.82 °F, 54.36 K
Boiling Point −182.96 °C, −297.33 °F, 90.19 K
Density 0.001308 g cm−3
Chemical properties
Atomic radius (non-bonded) 1.52 Å
Covalent radius 0.64 Å
Oxidation number or states −1, −2
Ionization energy (kJ mol−1) 1st 2nd 3rd
1313.94 3388.67 5300.47
Electron affinity 140.976 kJ mol−1
Electronegativity 3.44 (Pauling scale)
Molar heat capacity
 29.378 J mol−1 K−1
CAS number 7782-44-7

The bond dissociation energy of O2 is high (496 kJ mol−1). Therefore, reactions involving such dissociation require high energy of activation.

Physical Properties

Dioxygen (molar mass = 16 gm/mol) is a colourless, odourless, tasteless gas that occurs in two allotropic forms dioxygen (molecular weight = 32 g/mol) and ozone gas (molecular weight = 48 g/mol).

The colorless, orderless dioxygen molecule is paramagnetic with two unpaired electrons. Dioxygen is fairly soluble in water and highly soluble in organic solvents like acetone and benzene and forms weak charge-transfer complexes.

Dioxygen is a pale blue liquid but forms a blue solid crystal lattice after converting to a solid. The gaseous oxygen molecule colorless but in liquid or solid form, a single photon collides with other molecules and excites both. Absorption of electromagnetic spectrum radiation in the red region to the green visible region gives the observed blue color.

Chemical Properties

Oxygen (meting point = − 229 °C and boiling point = − 183 °C) has very high ionization energy among the group 16 chemical elements. It has a very low metallic character and is commonly known as a nonmetal.

With the increasing atomic number in group 16, the resistance of the elements decreases. Oxygen and sulfur are insulators, selenium and tellurium are semiconductors, but polonium is a metal that conducts electrical energy.

Bonding in Oxygen Molecule

The electronic configuration of oxygen is 1s2 2s2 2p4. Therefore, the element is two electrons short of the next noble gas configuration. Therefore, it achieves these two bonding electrons by gaining two electrons from electropositive elements or by making two single covalent bond or one double bond with the same or other elements.

In an O2 molecule, the two oxygen atoms share two electrons to form two covalent bonds (one sigma and one pi bond). The sigma bond in the oxygen molecule is formed by the axial overlap of 2p atomic orbitals and the pi bond is formed by side on overlap of 2p atomic orbitals.

Oxygen has very high electronegativity and electron affinity in favor of the formation of crystalline solid ionic compounds.

Mg → Mg+2 + 2 e
O + 2 e → O
Mg+2 + O−2 → MgO (Ionic crystal)

It commonly forms ionic compounds with alkali metals (lithium, sodium, potassium) and alkaline (beryllium, magnesium, calcium) earth metals.

Oxidation Number of Oxygen

The elemental oxygen generally shows a −2 oxidation number or state in a chemical compound due to the 2s2 2p4 outer electronic configuration. However, in peroxide (hydrogen peroxide) and superoxides, the oxidation number of oxygen is −1 and −½ respectively.

In F2O, the oxidation number of oxygen is +2 because fluorine is more electronegative than oxygen and shows a −1 oxidation number.

Interesting Facts About Oxygen

In learning chemistry, the outermost quantum shell consists of the 2s2 2p4 electronic configuration. The interesting fact about oxygen alone in the group16 or chalcogen family, they do not possess d-orbital. Therefore, oxygen only shows valency 2.

However, due to the presence of vacant d-orbitals other group 16 elements (sulfur, selenium, tellurium, and polonium) show 2, 4, and 6 valances for chemical bonding purposes.

Molecular Orbital Diagram of Oxygen

The valence shell electronic configuration of oxygen atom: 2s2 2px2 2py1 2pz1 with six electrons. Therefore, in the O2 molecule, we have to accommodate a total of twelve electrons in the molecular orbitals.

Molecular orbital diagram of oxygen (O2) molecule with atomic orbitals of oxygen atom

The molecular orbital configuration of the O2 molecule:

σ2s2, σ2s2, σ2pz2, π2px2 = π2py2, π2px1 = π2py1

From the above electronic configuration, the bond order of the O2 molecule = ½(8 − 4) = 2. Therefore, the O2 molecule contains a double bond where one is a sigma bond and the other is a pi bond.

In the ground state, the highest occupied molecular diagram of the O2 molecule shows the two electrons (parallel spin) in two pi-antibonding orbitals. It is said to be the triplet state.

However, the next higher state is one in which the electrons are spin-paired in pi-antibonding molecular orbitals. These states are termed singlet states. Normally a triplet to singlet transition is forbidden. Therefore, gaseous O2 is colourless.

Production Process

Oxygen element or gas molecule is obtained industrially by fractional distillation of liquid air.

In biochemistry, oxygen is produced as a byproduct during photosynthesis where plants take in carbon dioxide and water in the presence of sunlight.

6 CO2 + 6 H2O + photons → C6H12O6 (glucose) + 6 O2

During the presence of sunlight, carbon dioxide and water are used up by plants and many other microorganisms for the formation of glucose (carbohydrates) and oxygen.

In the laboratory, it may be prepared from different chemical compounds like hydrogen peroxide, potassium chlorate, potassium permanganate, etc. It can be produced by the catalytic decomposition of hydrogen peroxide over the platinum catalyst in nickel foil.

2 H2O2 → 2 H2O + O2

Alkaline Water Electrolysis

Electrolysis of 30 percent KOH solution with nickel electrodes produced oxygen in the anode and hydrogen in the cathode. The electrochemical reactions occurring at the cathode and anode are:

  • Cathode (reduction): 2 H2O → H2 + 2 OH
  • Anode (oxidation): 2 OH → ½ O2 + H2O
  • Overall reaction: H2O → H2 + ½ O2

Potassium hydroxide is preferred over sodium hydroxide due to the higher conductivity of potassium hydroxide solution.

Preparation of Oxygen from Potassium Chlorate

The thermal decomposition of potassium chlorate produces oxygen at 400° to 500 °C.

2 KClO3 → 2 KCl + O2

If we use manganese dioxide as a chemical catalyst, the reaction occurs at 150 °C. But this process also produces 3 percent of chlorine dioxide (ClO2).

The thermal decomposition of pure potassium permanganate in a vacuum at specific heat gives a very pure O2 molecule.

2 KMnO4 → K2MnO4 + MnO2 + O2

Singlet Oxygen

Singlet oxygen has been prepared in several ways:

  1. It can be prepared by irradiation of O2 in the presence of a sensitizer like fluorescein, methylene blue, etc.
  2. Singlet oxygen can also be produced during decomposition of ozonides or hydrogen peroxide.
    H2O2 + Cl2 → 2 H+ + 2 Cl + O2

We can reprent siglent O2 in fist transition state by O2. It has a sufficient lifetime to enter into chemical and biological reactions.

What is Oxygen Used For?

Oxygen is the third chemical in order of use in the industry after sulfuric acid and nitrogen. Therefore, nearly 100 million tonnes of O2 is consumed annually throughout the world.

  • It is largely used in metallurgy like steel making in blast furnaces and Bessemer converters.
  • It is also used in direct oxidation in many chemical processes.
  • The colorless, odorless, tasteless O2 gas molecule is used for making synthesis gas in chemistry.
  • In organic chemistry, it is used for oxidizing organic hydrocarbons like methane, ethane, ethylene, acetylene, etc.
  • It is an oxidizer for the fuels in rocket propulsion.

Uses in Our Animal or Plant

The processes that all living organisms perform to maintain their life are called life processes. Oxygen is an essential chemical element for the life process of all animal or plant bodies.

Oxygen in Animal Body

In the animal body, it is carried out by two metalloproteins hemoglobin and myoglobin in our body. Besides hemoglobin and myoglobin, there are two other dioxygen transport proteins hemocyanins and hemerythrin. Hemerythrin is found in marine invertebrates.

An adult human contains about 5 liters of blood. Each milliliter of blood contains 5000 million blood cells and each cell contains 0.25 million hemoglobin molecules. The red blood cells have a life span of 100 to 120 days. Therefore, one percent of hemoglobin molecules are replaced daily.

Oxygen Transport in Blood

Hemoglobin is essential for oxygen transport in our bodies. Myoglobin is engaged in the storage of O2 in muscle tissues and is used when necessary. The oxygen in our body is used in the biosynthesis of many compounds in the metabolic chain. Oxygen may convert some lipid-soluble molecules to water-soluble ones for excretion.

Oxygen in Plant Body

In plant leaves, gaseous exchange takes place by diffusion of oxygen molecules through stomata into the cells of the leaf. The direction of diffusing mainly depends on the environmental conditions and requirements of plants.

  • During the daytime, when photosynthesis occurs, carbon dioxide is rapidly taken up by plants while oxygen release is the major event. Therefore, during the daytime, O2 molecules diffuse in leaves and CO2 molecules diffuse out from leaves.
  • On the other hand during the night, the elimination of carbon dioxide from plants is a major event. Therefore, during the daytime, CO2 molecules diffuse in leaves and O2 molecules diffuse out from leaves.